1. Field of the Invention
The present invention relates to treatment of waste streams containing organic nitrogen compounds, and more particularly to ammonification of such streams by microbial degradation in a fluidized bed reactor.
2. Description of the Prior Art
Increasingly stringent goals for minimizing waste have necessitated the search for new technologies for waste reduction. With respect to streams of unacceptably high levels of organic nitrogen compounds, this need has been particularly notable with respect to streams that contain organic nitrogen compounds such as nitriles, especially dinitriles.
Nitriles, including mono-nitriles and dinitriles, are recognized as troublesome contaminants of waste water streams. Dinitriles, such as fumaronitrile and succinonitrile, are especially notorious for their acute toxicity and resistance to degradation. Note, for example, European patent application no. 87310689.2, publication no. 0 274 856, of Knowles. Moreover, degradation of dinitriles produces twice as much ammonia as does degradation of nitriles. The ammonia production has been viewed as a serious problem due to its toxicity and effects on pH.
Organic nitrogen compounds are found in many types of waste streams, often as by products in synthesis of organic compounds. For example, acrylonitrile (AN) is produced by an ammoxidation process in which propylene and ammonia are catalytically combined. During this process, by-products such as succinonitrile, fumaronitrile and maleonitrile are formed and end up in the stripper bottoms and so in the waste water along with some of the acrylonitrile. In addition, the waste water contains ammonium salts and organic acids as well. For example, a typical waste water stream from acrylonitrile production may include hydrogen cyanide, acetonitrile, acrolein, acrylonitrile, oxazole, propionitrile, methacrylonitrile, acetic acid, c-crotononitrile, allyl cyanide, t-crotononitrile, acrylic acid, cyanobutene, cyanobutadiene, pyrazine, cyanobutadiene, maleonitrile, fumaronitrile, cyanofuran, cyanopentadiene, cyclopentadiene, cyanopropanal, acrylamide, succinonitrile, maleimide, benzonitrile, cyanopyridine, methylbenzonitrile and cyanopentene, among others. Similarly, waste water containing organic nitrogen compounds is produced during the production of other nitrogen-containing compounds.
Various techniques have been employed for reduction of organic nitrogen compounds in waste streams by degrading such compounds to environmentally acceptable lower compounds, ultimately carbon dioxide and ammonia. For example, biological degradation processes (see, for instance, the European patent application of Knowles noted above), activated sludge systems (see, for instance, U.S. Pat. No. 3,940,332 to Kato et al.) and deep-well injection methods have been used. However, such techniques have several significant drawbacks. The presence of nitriles in the waste has been found to exert a strong inhibitory effect on activated sludge systems. In addition, production of ammonia in the aqueous streams not only results in toxicity (and can kill the microorganisms required for effecting degradation) but also raises the pH of the effluent to unacceptable levels, requiring the addition of acids or buffers. Generally, ammonia levels of about 300 mg/l or more, especially about 500 mg/l or more, are troublesome unless the medium is buffered.
Moreover, such techniques are not as effective as desired. For example, the noted references report efficacy (that is, reduction of chemical oxygen demand (COD)) in the range of only about 71% (Kato) to about 7% (Knowles) for treatment of acrylonitrile bottoms, and even then such efficacy is reported only for low organic nitrogen content streams. For example, Knowles describes an AN stripper column bottoms waste having a TKN of only 180 mg/l Knowles reports high efficiency ratings only for streams of relatively low levels of organic nitrogen compounds. Thus, techniques for more effective degradation are desired, especially for streams of TKN above 300 mg/l especially above about 500 mg/l
Fluidized bed reactors (FBRs) utilizing immobilized bacteria technology (IBT) contain a fluidized bed of particulate solids as a biocarrier supporting microorganisms capable of biodegrading certain compositions. FBRs achieve expansion of the biocarrier bed by recycling the waste water being treated upward through the reactor. Use of FBRs employing IBT has been reported in certain waste water treatment processes. For example, U.S. Pat. No. 4,009,099 to Jeris describes a method for treating waste water with an FBR using IBT. However, that patent describes the use of an FBR for converting ammonia nitrogen in waste water to an oxidized form. The patent is not directed to degradation of organic nitrogen compounds to ammonia, but to nitrification of ammonia to NO.sub.3, which would then require denitrification. Thus, it is not applicable to treatment of nitrile streams.
A recent article in Water Environment Research, Vol. 66, pp. 70-83 (January/February 1994), entitled, "Laboratory-Scale Evaluation of Aerobic Fluidized Bed Reactors for the Biotreatment of a Synthetic, High-Strength Chemical Industry Waste Stream," describes the use of an FBR to treat waste streams. However, this article by D. E. Edwards, W. J. Adams and M. A. Heitkamp (a co-inventor of the present method), is directed to treatment of very low organic nitrogen content streams and contains no indication that such method would prove effective in degrading significant amounts of organic nitrogen compounds.
An article of P. M. Sutton et al. published in the 45th Purdue Industrial Waste Conference Proceedings(1991 Lewis Publishers, Inc., Chelsea, Mich.), at pages 751-758, "Biological Treatment of Wastewater from Byproduct Coking Operations: An Innovative Approach", describes a treatment process that requires external pH control and recognizes that performance may be affected negatively by high levels of ammonia nitrogen in the feed. U.S. Pat. No. 4,059,492 to Hamweiler et al. discloses a process for purification of waste from acrylonitrile production. However, conventional techniques generally do not degrade the organic nitrogen compounds in the waste streams to the degree desired and often they require exogenous buffering such as by the addition of phosphoric acid because of the production of ammonia. This acidity may then make ammonia stripping downstream difficult and limit the ability to recycle or reuse water. Accordingly, expensive techniques to deal with the added acid or to remove the ammonia in the presence of the acid are employed.
Thus, conventional techniques involve long, complex development of specific consortia of bacteria or, as in Jeris, a long, slow nitrification and denitrification process. Such processes therefore tend to be complex, slow and expensive, and involve use of exogenous buffering with its attendant complications in later ammonia removal. Moreover, despite these accommodations, the processes still do not tend to degrade the contaminants to the degree desired.
Accordingly, industries that must deal with disposal of aqueous waste streams containing organic nitrogen compounds are still searching for improved methods of more effectively degrading the organic nitrogen compounds in the waste water (that is, that achieve a greater reduction in COD) and that are environmentally compatible.